Energy storage devices such as batteries that utilize the oxidation and reduction reactions of an alkali metal are known. Such xe2x80x9clithium ion cellsxe2x80x9d include secondary cells which use a carbon material that can be doped and undoped with lithium ions as a negative pole and which use a complex oxide of lithium and a second metal as a positive pole. Lithium ion cells are compact and lightweight, and have a high energy density. Accordingly, the use of lithium ion cells as secondary cells in portable electronic devices has expanded rapidly. Concurrently, there has been an escalating demand for improved performance, e.g., an increase in the energy density and an increase in the discharge current, etc., in lithium ion cells in order to achieve a further improvement in the function of such portable electronic devices.
The decreased size of such energy storage devices has resulted in the presence of highly energetic active substances in a small, confined volume. As a result, large amounts of energy can be released when electrodes short-circuit or otherwise fail as a result of, for example, piercing and compression that may cause a battery to ignite and catch fire. As the cell capacity has increased, there has been a strong demand for an improvement in battery safety.
Previous attempts to increase safety have been directed to changing electrode construction or changing the active substance. Other studies have focused on additives to the electrolyte solution that increase the safety of the batteries. Thus, for example phosphorus and fluorine compounds such as triphenyl phosphate and fluoro-ethers have been dissolved in the electrolyte solution to improve battery safety. However, these compounds may be subject to oxidation-reduction by the electrodes, or may react with the electrodes so that the capacity is lowered. Although safety is improved when the amounts of additives are increased, battery performance deteriorates. Accordingly, it has been difficult to realize increased safety without causing a deterioration of conventional battery characteristics.
There is thus a continuing need for improved cell capacity, charge-discharge rate, and charge-discharge cycle.
There is also a continuing need for an improved battery with an improved safety profile that does not deteriorate over time.
One object of the present invention is to provide an electrolyte system for an energy storage device that is extremely safe and has superior charge-discharge characteristics.
Another object of the present invention is to provide an electrolyte system of high conductivity and low viscosity for improving the discharge capacity of a secondary cell battery.
Another object of the present invention is to provide an electrolyte system that is chemically and electrically stable with respect to the positive pole and negative pole of the secondary cell battery.
In accordance with the present invention, an electrolyte system comprises a non-aqueous electrolyte solution including a non-aqueous solvent and a salt, and a flame retardant material that is a liquid at room temperature and pressure and that is substantially immiscible in the non-aqueous electrolyte solution.
The non-aqueous electrolyte solution is formed by dissolving a salt, preferably an alkali metal salt, in a non-aqueous solvent. The non-aqueous solvent is a polar aprotic organic solvent which readily dissolves alkali metal salts, and which is resistant to being electrolyzed by oxidation-reduction at the battery poles. The non-aqueous solvent preferably includes a cyclic carbonate and/or a linear carbonate, wherein the cyclic carbonate preferably contains an alkylene group with 2 to 5 carbon atoms, and the linear carbonate preferably contains a hydrocarbon group with 1 to 5 carbon atoms. Preferred electrolytes include LiPF6, LiBF4, LiOSO2R1, 
(in the above formulae, R1 through R8 indicate perfluoroalkyl, alkyl or aryl groups with 1 to 6 carbon atoms, which may be the same or different groups). The concentration of the electrolytes in the non-aqueous electrolyte solution is preferably between about 0.1 to 3.0 moles/liter, and more preferably between about 0.5 to 2.0 moles/liter.
The flame retardant material is a liquid at room temperature and pressure and is substantially immiscible in the non-aqueous electrolyte solution. Preferably, the flame retardant material is a halogen-containing compound. Preferred halogen-containing compounds contain perfluoroalkyl groups or perfluoroether groups. The halogen-containing compound is present in an amount by weight of non-aqueous solvent in a range of from about 1 to about 99 wt %, preferably from about 1 to about 70 wt %, even more preferably from about 10 to about 60 wt % and even more preferably from about 20 to about 40 wt %.
An energy storage device in accordance with the present invention comprises the disclosed electrolyte system, an electrode assembly including a first electrode member, a second electrode member, a separator member physically and electrically separating the first electrode member from the second electrode member but capable of allowing ionic conductivity between the first electrode member and the second electrode member through the non-aqueous electrolyte solution, and a casing enclosing the electrode assembly and the electrolyte system. In a energy storage device, such as a battery, the first electrode member is a negative electrode containing a material selected from the group consisting of lithium metal, a lithium alloy, a carbon material that can be doped and undoped with lithium ions, a metal oxide that can be doped and undoped with lithium ions, and silicon that can be doped and undoped with lithium ions, the second electrode member is a positive electrode containing a material selected from the group consisting of complex oxide of lithium and a transition metal, and a complex oxide of lithium, transition metal and a non-transition metal, and the separator member is a resin containing a polymer.
A method of making a energy storage device in accordance with the present invention comprises providing an electrode assembly including a first electrode member, a second electrode member and a separator member physically and electrically separating the first electrode member from the second electrode member but capable of allowing ionic conductivity between the first electrode member and the second electrode member, placing the assembly in a casing, and filling the casing with the electrolyte system of the present invention by first, filling the casing at least partially with the non-aqueous electrolyte solution, waiting a period of time sufficient for the non-aqueous electrolyte solution to penetrate one or more pores of the electrode assembly, and then adding the flame retardant material to the casing.
In practice, the two phase electrolyte system of the present invention realizes significant advantages over the prior art. Because the halogen-containing compound is substantially immiscible in the non-aqueous electrolyte solution, there is little ingress of the halogen-containing compound into the regions within the casing occupied by the non-aqueous electrolyte solution, i.e. in the casing regions separating the positive and negative poles and defined by the separator. As such, there is little interference by the halogen-containing compound in electrochemical reactions occuring at the poles and in the non-aqueous electrolyte solution. Further, the secondary cell is superior in terms of initial capacity and cycle characteristics. Moreover, the halogen-containing compound of the present invention interferes and/or inhibits combustion reactions that may result upon piercing or compression of the battery. Accordingly, the energy storage device of the present invention exhibits enhanced safety over prior art secondary cells.